Fatigue Tab Best Practice
1.Fatigue analysis/post-processing should typically be performed from a previously ran dynamic analysis whose general results have already been checked for validity (i.e. you should use the Previous fatigue Analysis Option). Note that once fatigue post-processing has been completed for the first time and afterwards you wish to change some general fatigue parameters, such as the S-N curve or the pull/cycle time, then you can quickly re-run the fatigue calculations using the Restart option provided the Stress History Timetraces have not been deleted from the original fatigue run (see setting at the bottom of the Fatigue tab).
2.It is recommended to use the Combined Stress option for fatigue calculations as this is a linear sum of axial and bending stresses, which realistically accounts for sign reversals in bending curvatures/axial strains unlike the Von Mises stress option.
3.Normal Lay type fatigue accumulation as a result of pipe pulls starts from the governing vessel weld downwards, meaning that any damage above the governing vessel weld is not accounted for. Therefore if you wish to capture all fatigue along the line then you should position just one vessel weld directly at the vessel end of the line. Catenary welds should be placed a pull length apart.
4.For Staged Operations or Hybrid Staged Operations type fatigue the selection of which installation stages to include for dynamic/fatigue analysis is important. Generally static analysis covers a large number of the installation stages in an operation, however it is not necessarily feasible to analyse the same number of stages dynamically and so a selection of the governing stages must be made with respect to dynamic/fatigue analysis. While regular wave loading is not appropriate for fatigue design it can be used to screen fatigue across a lot of stages so as to establish the governing ones required for full random sea dynamic/fatigue analysis. The selection of a tracked reference point is also important for these fatigue types and it must correspond to a point that moves down along the catenary across the various stages in the operation (e.g. in the case of A&R the weld point of the pipe head is a good selection for the tracked reference point as the head is lowered to the seabed during the operation). For reference the Conference of Floating Structures for Deepwater Operations (University of Strathclyde) 2009 paper, “Fatigue Analysis of Staged Operations”, outlines a case study for successfully applying the Staged Operations fatigue analysis in PipeLay to a sample abandonment scenario.